1. Field of the Invention
The present invention relates to magnetic recording media, methods for manufacturing recording media and magnetic recording apparatus, in particular, to magnetic recording media having high recording density, methods for manufacturing the recording media and magnetic recording apparatus such as hard disk drives in which the high-density recording media are equipped.
2. Description of the Related Art
Hard disk drives (HDDs) have been expanding their application scope from the first computer related application to various other applications, such as home video recorder and car carrying navigation system applications as magnetic recording systems for recording and reproducing information. The expansion is due to their advantage such as high data access speed and high data storage reliability in addition to their high recording capacity performance with low cost. Requirements for HDDs having larger recording capacity have been increased with the expansion of the HDD application scope. Replying to the requirements, large capacity recording technology has been advanced by increasing recording density of the magnetic recording media.
With increasing recording density of the magnetic recording media of HDDs, the recording bit sizes and the diameters for the magnetization reversal units became very small. As the result, thermally decreasing phenomena of recorded signal magnetization and the recording and reproducing performance by thermal fluctuation effect caused by the very small magnetization reversal units became notable. Furthermore, noise signals which appear at boundary regions between recording bits became large as a result of decreasing recording bit to a very small size, and the noise became to give a large influences upon the signal to noise ratio. Therefore, in order to attain further high recording density, it is required to stabilize thermal stability of the recorded signal magnetization at one hand and to attain low noise characteristics at high recording density at the other hand.
To decrease magnetic recording medium noise, size of magnetic crystalline grains constructing recording-layer have been made smaller up to now. For example, magnetic crystalline grains of Co—Cr magnetic-layer of widely used magnetic recording media were made small by adding small amount of Ta or B (refer to Japanese Patent Laid-open Applications Nos. HEI 11-154321 and 2003-338029), and by precipitating nonmagnetic Cr by heat treating at appropriate temperature (refer to Japanese Patent Laid-open Applications Nos. HEI 3-235218, and HEI 6-259764). Recently, a method for obtaining magnetic recording layer having so called granular structure obtained by adding oxides such as SiOx to the magnetic layer was applied. In the granular structured magnetic layer, nonmagnetic grain boundary material enclose magnetic crystalline grains (refer to Japanese Patent Laid-open Applications Nos. HEI 10-92637, and 2001-56922).
These methods, however, cannot control the crystalline grains of the magnetic-layer and the under-layer by going back to the nucleation processes for the crystalline grains of the under-layer and the magnetic recording layer. These methods control an average magnetic crystalline grain diameter and grain boundary regions merely by choosing combination of raw materials, the raw material composition, or by choosing of depositing conditions. When the crystalline grain diameters in the under-layer are tried to make smaller, the crystalline quality and crystal orientation degree of the grains in the under-layer are degraded, and the degraded crystalline quality of the under-layer grains gives influence upon the formation of magnetic crystalline grains.
Actually it was found that the magnetic-layer prepared using this procedure showed distributions of broad grain size and grain boundary width. Magnetic recording media decreasing the average grain size of the magnetic crystalline grains to 5 nm showed poor thermal fluctuation durability. Very small grain diameter components unstable to thermal fluctuation were included at large fraction. Then it was difficult to attain further high recording density using this method.